The present invention relates to winches and winch drums and in particular to winch drum surface finishes.
Winches are commonly used in marine applications to control the length or slackness of a rope acted on by the rotatable drum of the winch. An important feature of the winch drums of such winches is the finish applied to the rope-gripping surface of the drum. Different types of finish produce different grip characteristics. However, because a given finish will always cause rope wear, in practice the choice of a particular finish for a given application will often represent a compromise between grip and wear.
One conventional finish is provided by shot-peening. In this process, small hard objects (typically ceramic grit) are fired at the grip surface of the winch drum. The result is a random arrangement of relatively fine (in relation to the dimensions of the drum) peaks and troughs on the grip surface of the winch drum.
Peened finishes provide good gripping characteristics and controlled letting-off of rope, but have a tendency to promote premature rope wear. In addition, the quality of the peened finish can be variable. That is, peened finishes are susceptable to variations caused by e.g. changes to the velocity and direction of the peening objects. Subsequent processing can also affect the finish. For example, chrome plating onto a peened brass surface will tend to xe2x80x9csoftenxe2x80x9d the finish of the brass surface. Thus there are many factors which can make it difficult to maintain the reproducibility of peened finishes.
It is also difficult to quantify the essential parameters of any particular finish (meaning those parameters which determine grip and wear) because of the essentially random nature of the arrangement of peaks and troughs. This can be a particular disadvantage if it is desired to investigate systematically the effects of different finishes on rope grip and rope wear, e.g. by varying systematically the parameters of the finish.
Another known finish consists of having an arrangement of vertical slots or ribs on the grip surface of the drum. The slots or ribs are generally parallel to the axis of rotation of the drum, i.e. perpendicular to the winding direction of the rope. However, the grip provided by this type of finish tends to be too strong so that when controlled release of the rope is attempted from the winch, large quantities of rope are let off at a time rather than an even, slow feed-off.
FR-A-1214874 shows an auxiliary windlass for a fishing boat where frusto-conical projections are arranged on the drum in planes normal to the axis of rotation of the drum, with gap planes between them. The projections are to mesh with a fishing net being drawn onto or leff off from the boat. They do not engage the rope.
With an aim of providing a rope-gripping surface finish whose parameters are both measurable and adjustable, while also providing good rope control and acceptable rope wear, the present invention provides a winch drum in which the finish of a rope-gripping surface is provided by a regular staggered array of protuberances which are of generally pyramidal shape. Note that the term xe2x80x9cpyramidalxe2x80x9d is not to be construed as being limited to square-based pyramidal shapes, i.e. tetrahedra and other polygonally-based pyramidal shapes are also included. Preferably, circumferentially nearest-neighbour protuberances are staggered in the axial direction of the drum; in other words, the only array of protuberances is a staggered one. If as is also preferred there are not circular paths normal to the axis of rotation of the drum (the hoop direction of the drum), the effect is that a rope wound in the hoop direction around the drum experiences spaced axial forces on opposing and alternate sides so that the rope tends to follow to some extent a sinusoidal or otherwise snaking path. Faces of the pyramidal shapes may be concavely curved so that respective, radially innermost, edges of adjacent, corner-sharing protuberances form continuous, smooth junctions which prevent a rope wedging fast in the junctions. The pyramidal shapes may be produced by machining on a CNC lathe, but other production techniques, such as casting or pressing, can also be used. For convenience the pyramidal shapes will hereafter be referred to simply as pyramids.
By varying the height, base shape and size (i.e. the characterizing parameters) of the pyramids, it is possible to exert a high degree of control over the surface finish and its effects. In addition these characterizing parameters are readily quantifiable so that it is possible to correlate measurable characteristics of the surface finish with the performance of the finish. This permits a methodical approach to the optimization of the rope-gripping surface finish to be undertaken.
Furthermore, compared with solely peened finishes, quality control of the regularly patterned finish is much improved, as a high degree of control can be exerted over the machining or other operation which produces the finish.
In a preferred form of the rope-gripping surface, the protuberances are diamond-based pyramids, the regular pattern being formed from a tessellation of the diamond bases of the pyramids. Desirably the pyramids are arranged so that the long direction of the base of the pyramids is aligned to the hoop direction of the winch drum. It is found that such an arrangement can provide a good compromise between rope control and rope wear.